Aircraft lubrication and scavenge systems provide oil for lubrication of aircraft components. The oil is scavenged through the use of pumps which return the oil to a reservoir for recirculation. The oil scavenged by the pumps can have a high air content in an air-oil mixture. Before recirculating the oil from the main reservoir it is preferred to remove the air from the air-oil mixture to avoid plugging and slugging, e.g. pressure pulsations and slugs of air, then slugs of oil in the fluid provided to the aircraft components. These pressure fluctuations can cause damage to the aircraft components. Cooling with oil is also more effective than with an air-oil mixture, as in an heat exchanger, for example.
It is known to employ gear pumps for scavenging the air-oil mixture in aircraft lubrication and scavenge systems before returning the oil to a reservoir for recirculation. In these known systems an air-oil separator is employed downstream from the gear pump for removing air. These gear pumps are required to pump various air-oil ratios at pressure ratios ranging anywhere from 1 to over 50. At the higher pressure ratios the carryover and leakage of air from discharge to inlet of the pump degrades the inlet vacuum and partially fills the potential volume of the pump. One of the reasons for high discharge pressure in these pumps is the necessity to run the two phase mixture through an air-oil separator.
It has been recognized that in gear pumps having intermeshing gears which are used to pump aerated liquid, liquids containing gases or emulsions of liquids and gases, that the liquids are centrifuged to the perimeter of the gears and the gaseous phase becomes trapped in a void formed at the delivery side of the pump in the lee of the intermeshing gears. To prevent the entrapped gas from becoming highly compressed and being forced through the intermeshing gears back to the inlet side of the pump at extremely high velocities, it is proposed in U.S. Pat. No. 4,480,970 to provide a vent pipe at the outlet side of the pump close to the nip of the intermeshing gears. However, this known arrangement does not prevent return of a portion of the air to the reservoir or eliminate the need for a separate air-oil separator downstream of the gear pump.
Rotary positive displacement type pumping apparatus for centrifugal separation of liquid and gaseous fluids are, per se, known as disclosed in U.S. Pat. Nos. 3,137,234 and 3,242,867. U.S. Pat. No. 2,619,911 discloses a pump with intermeshing gears for centrifugal separation. An addition of supercharged liquid is employed to forcibly exhaust air and accumulated vapor from the inner portion of the interdental space or pocket.
U.S. Pat. Nos. 1,477,850; 2,180,771; and 2,458,452 disclose examples of serially arranged pumps wherein the downstream pump is of less capacity than the upstream pump and also the use of relief valve between the pumps in order to divert excess fluid flow. U.S. Pat. Nos. 3,526,470; 4,631,009; and 4,697,995 further show the state of the art.